This invention relates generally to seismic surveying methods, and more particularly to an improved seismic surveying method of using a plurality of vibratory seismic sources.
Seismic vibrators have been used for many years on land to acquire seismic data and many companies have ongoing efforts to utilize similar sources in marine environments. The geophysical and environmental benefits of using these types of seismic sources are well known.
When seismic data is acquired utilizing a plurality of vibratory seismic sources, the vibrators are conventionally organized as a travelling source array. The vibrators are typically placed around or along a source point (also referred to as a xe2x80x9cvibrator pointxe2x80x9d or a xe2x80x9cvib pointxe2x80x9d) with a particular separation distance, such as 40 meters. The vibrators then generate a certain number of sweeps that are received by a plurality of seismic sensors, recorded and stacked (i.e. combined) to produce a seismic data trace for each particular source point/receiver point pairing. The vibrators then travel as a group to the next source point where they are used in a similar manner.
There are several known problems with acquiring seismic data using seismic vibrators, however, including the need to acquire large numbers of relatively-long records for each source point/receiver point pair to produce seismic data having a sufficiently high signal to noise ratio. Other known problems with seismic data acquisition using seismic vibrators include harmonics, ground coupling differences, baseplate flexures, and source array effects.
Efforts have been made to address these problems, and one promising approach has been the simultaneous use of multiple vibrators at different source points, with each vibrator producing separable, encoded sweeps. One method using this approach, referred to as the High Fidelity Vibroseis Source (xe2x80x9cHFVSxe2x80x9d) method, has been developed by Mobil Oil Corporation and Atlantic Richfield Company and is described in U.S. Pat. Nos. 5,550,786 (Aug. 27, 1996); 5,570,833 (Dec. 30, 1997); 5,715,213 (Feb. 3, 1998); and 5,721,710 (Feb. 24, 1998), all incorporated herein by reference. The HFVS method was developed primarily to improve the fidelity of vibroseis data.
The HFVS method may be described, in principle, as comprising the following steps:
1. Measuring the vibrator motion S for each vibrator and each sweep, typically using an accelerometer mounted to the vibrator base-plate. The measured signal S is related to the true vibrator output U and a minimum phase transfer function T1. In the frequency domain, the equation describing the measured signal S is: S=U*T1.
2. Recording the seismic data R. This seismic data represents the multiplication in the frequency domain between the earth reflectivity E, the vibrator output U and a minimum phase transfer function T2: R=U*T2*E.
3. Obtaining the earth reflectivity at the vibrator location by multiplying the record R with the inverse of the vibrator motion U: R/U=T1/T2*E
For an array of 4 vibrators, V1, V2, V3, and V4, sweeping simultaneously, the geophone response R is described in the frequency domain by the following linear equation: R=m11*h1+m12*h2+m13*h3+m14*h4. This equation contains 4 unknowns, h1, h2, h3, and h4 (the earth response at the vibrator positions V1, V2, V3, and V4) and contains the known values R (the geophone response) and m11, m12, m13, and m14 (the measured signals).
The unknowns h1, h2, h3, and h4 can be determined if another 3 sweeps are generated at the same locations and if the sweeps are encoded in such a way that the measured signal matrix is invertable. The system of linear equations is:
R1=m11*h1+m12*h2+m13*h3+m14*h4
R2=m21*h1+m22*h2+m23*h3+m24*h4
R3=m31*h1+m32*h2+m33*h3+m34*h4
R4=m41*h1+m42*h2+m43*h3+m44*h4
In matrix notation, this can be written as:
R=mxc3x97h
where       R    =          [                                                  R              1                                                                          R              2                                                                          R              3                                                                          R              4                                          ]        ,      xe2x80x83    ⁢      m    =          [                                                  m              11                                                          m              12                                                          m              13                                                          m              14                                                                          m              21                                                          m              22                                                          m              23                                                          m              24                                                                          m              31                                                          m              32                                                          m              33                                                          m              34                                                                          m              41                                                          m              42                                                          m              43                                                          m              44                                          ]        ,      xe2x80x83    ⁢            and      ⁢              xe2x80x83            ⁢      h        =                  [                                                            h                1                                                                                        h                2                                                                                        h                3                                                                                        h                4                                                    ]            .      
The typical implementation of the HFVS method in the field involves one array or group of vibrators, often four, spread out on an equal number of consecutive stations or source points. The vibrators sweep a certain number of sweeps, let say N (N being greater than or equal to the number of vibrators) at the same locations. The sweeps have the same frequency content but the phase is differently encoded to assure that the matrix M is invertible. After N sweeps, the vibrators move up a number of stations equal to the number of vibrators and repeat the sequence.
This implementation of the HFVS method has typically performed well in areas with shallow targets and good signal to noise ratios. For deeper targets or poor signal to noise areas, the standard implementation of the HFVS method may not perform well. The number of traces required for each source/receiver pair (the xe2x80x9cfoldxe2x80x9d) is also quite high, making acquisition of seismic data using this method relatively expensive.
It is therefore desirable to implement an improved method of acquiring seismic data using a plurality of vibratory seismic sources that overcomes problems exhibited by prior art seismic data acquisition methods.
An object of the present invention is to provide an improved method of acquiring seismic data using a plurality of vibratory seismic sources.
An advantage of the present invention is that for the same acquisition effort and expense, seismic data having a higher signal to noise ratio may be obtained.
Another advantage of the present method is that if coherent noise in the seismic data is band limited, it may be attenuated only in a particular frequency range, leaving the remaining frequency components of the seismic data unaffected.
The present invention provides an improved method of seismic surveying using a plurality of vibratory seismic sources, the method including the steps of:
deploying at least one seismic sensor;
deploying a plurality of vibratory seismic sources at different source points;
simultaneously actuating said seismic sources;
acquiring seismic data attributable to said seismic sources using said seismic sensor;
redeploying said seismic sources so that at least one of them is positioned at a source point previously occupied by another of them;
simultaneously actuating said redeployed seismic sources;
acquiring seismic data attributable to said redeployed seismic sources using said seismic sensor;
decomposing said acquired seismic data into components attributable to each said seismic source; and
stacking together components attributable to seismic sources located at a common source point.
The invention and its benefits will be better understood with reference to the detailed description below and the accompanying drawings.